1. Field of the Invention
This invention relates to a partially crosslinked olefinic thermoplastic elastomer composition having excellent film formability and being useful as a waterproof sheet.
2. Description of the Prior Art
Rubber products generally have found extensive use in applications which require elasticity and flexibility. Molding of the rubber products, however, does not lend itself to mass production of process problems such as the need for vulcanization. rubber substitutes have therefore been investigated which can be molded without vulcanization and have rubber-like properties.
Flexible plastics such as flexible vinyl chloride resins, ethylene/vinyl acetate copolymer and low-density polyethylene are known as such materials having rubber-like properties. They have good moldability and excellent flexibility, but have the defect of poor heat resistance, mechanical strength and repulsive elasticity (rebound) which greatly restrict their utility.
Attempts have also been made to increase the heat resistance and mechanical strength of such flexible plastics by mixing them with plastics having high melting points such as high-density polyethylene or polypropylene. Mixing of such high-melting plastics, however, degrades the inherent flexibility of such flexible plastics. Moreover, molded articles of good quality cannot be obtained from such mixtures because sinks and other imperfections occur during extrusion or injection molding.
In recent years, so-called thermoplastic elastomers have attracted attention as materials having properties intermediate between those of vulcanized rubbers and flexible plastics.
As such a thermoplastic elastomer, the present inventors previously proposed a thermoplastic elastomer obtained by dynamically heat-treating a peroxide-crosslinkable olefinic copolymer, a peroxide-crosslinkable polyolefin resin, an organic peroxide, a crosslinking aid and a polyolefin resin not crosslinkable with the peroxide (Japanes Laid-Open Patent Publication No. 71739/1980).
This thermoplastic elastomer has excellent heat resistance, tensile properties, weatherability, flexibility and rebound, and can easily be molded into articles free from flow marks by extrusion or injection molding. Particularly, it can be molded very satisfactorily into large-sized and thick articles. Its film formability is still not satisfactory enough, and it cannot fully satisfactorily find applications as a surface sheet of interior trimming materials of automobiles, for example floors, walls, ceilings, inner trunk walls, or seat backs, nor can a waterproof sheet be obtained from it which meets the standards of vulcanized rubbers in "Polymer Roofing" stipulated in "JIS A-6008".